1. Field of the Invention
The present invention relates to activated carbon honeycomb catalyst beds and, more particularly, to methods and systems for regenerating mercury loaded activated carbon honeycomb catalyst beds.
2. Technical Background
Emissions of hazardous materials and/or heavy metals such as Hg, Ni, Cr, Cd, Co, Pb, V, Se, Be, As, Zn, etc. have become environmental issues of increasing importance because of the dangers to human health. Of these metals, mercury is a trace element of particular concern, because during coal and municipal solid waste combustion, most of the mercury present in coal and municipal solid waste is transferred into the vapor phase due to its high volatility. It is also known that mercury can be transformed to a potentially toxic species (methylmercury) under natural conditions found in the environment. Mercury that is emitted to the atmosphere can travel thousands of miles for before being deposited to the earth. Further, studies have also shown that mercury from the atmosphere can be deposited in areas near an emission source.
Coal-fired power plants and medical waste incineration are major sources of human activity related mercury emission to the atmosphere. It is estimated that 48 metric tons of mercury are emitted from coal-fired power plants in the US every year and 1500 metric tons of mercury are emitted from coal-fired power plants worldwide. Currently available pollution abatement technologies are not capable of effectively controlling elemental mercury emissions particularly from flue gas emissions in the utility industry. Once discharged to the atmosphere, mercury persists in the environment and creates long-term contamination problems.
Among current state of the art mercury remediation technologies, adsorption on sulfur-impregnated activated carbon has shown some promise with removal of 50-90% mercury in flue gases depending on reaction conditions. Sulfur is introduced into activated carbon by impregnating with different forms of sulfur such as elemental sulfur, carbon disulfide, hydrogen sulfide, or sulfur dioxide. This honeycomb substrate based technology can significantly reduce the amount of hazardous waste. Additionally, the honeycomb based technology can enable the reuse of fly ash since it is not mixed with the active carbon powder. Still further, fixed bed honeycomb technology does not require an expensive carbon powder collecting system and can be placed at the end of the existing flue gas waste treatment system, which allows retrofitting to the existing treatment system. Thus, the actual cost to the power plant can be reduced further.
Since it is undesirable to utilize a mercury removal system occupying a relatively large space, a fixed bed honeycomb system may require frequent changing or replacement of the honeycomb sorbent in order to maintain adequate mercury removal efficiency. Such frequent replacement can result in an interruption of the system operation and added cost. Accordingly, there is a need in the art for an efficient method of regenerating a mercury loaded honeycomb sorbent catalyst bed.